It is well known in the art that a substantial portion of the emissions generated by a solid fuel combustion apparatus are themselves combustible. Conventional solid fuel burning apparatus are extremely inefficient because a substantial portion of the solid fuel's combustible energy is sent up the stack in the form of creosote, oils, tars, heavy combustible gases and smoke (pollution). Therefore, by ensuring more complete combustion of these emissions, the efficiency of the combustion apparatus is greatly improved and the quantity of noxious emissions is greatly reduced.
A common approach to increasing combustion efficiency is to establish secondary combustion. In a conventional wood burning stove the combustion of the solid fuel is generally referred to as primary combustion (“PC”). The region in which primary combustion takes place is referred to as the primary combustion zone (“PCZ”). Typically primary combustion occurs in the lower region of the firebox, on or near the solid fuel situated on the hearth or floor of the appliance. Combustion of the emissions of the PC is referred to as secondary combustion (“SC”), and the region or regions in which SC occurs are referred to as secondary combustion zones (“SCZ”). SC generally occurs above, beyond and “downstream” from the solid fuel PCZ location.
Prior art indicates that many attempts have been made to improve the combustion of solid fuel emissions by SC. These attempts can be divided largely into two groups: combustion apparatus with combustion catalysts and combustion apparatus based on staged primary and secondary combustion.
It is known in the art that the ignition point of the emissions created by solid fuel is about 537° C. (1000° F.). Traditional catalytic converters are typically inserted in the upper region of the firebox to bring the ignition temperature down a few hundred degrees and newer versions are now able to ignite flue gases in the 260° C.-315° C. (500-600° F.) range. Prior art such as “Advanced Techniques for Wood Log Combustion Emission Comparisons”, by Øyvind Skreiberg of the Norwegian Institute of Technology, Institute of Thermal Energy and Hydro Power, disclosed at Comett Expert Workshop, “Biomass Combustion”, May 1994) discloses that catalytic converters, however, can lead to an increase in NOX emissions. One further disadvantage of catalytic converters is that they are a consumable device—the coating that acts as the catalyst wears off with use.
Skreibeg also defines that staged combustion comprises: the separation of the gasification chamber (referred to hereinafter as the primary combustion zone, PCZ) and the additional combustion chambers (referred to hereinafter as secondary combustion zones, SCZ's); the use of downdraft combustion (referred to hereinafter as secondary combustion) to ensure good mixing of air and combustible fuel emissions; and good insulation of the combustion chambers as well as some preheating of the combustion air, both of which ensure a high combustion temperature. Gasification is the thermo-chemical process of converting the solid fuel into gaseous products such as oil, tar and other heavy combustible gases and occurs as the solid fuel heats up, breaks down and combusts. Downdraft combustion is created using secondary air sources which create turbulent air within the SCZ. It is well known in the art that SC occurs more readily in a turbulent air environment since turbulence lends itself to better mixing of oxygen and heavy particulates (emissions) and thereby promotes SC. It is also known in the art that the use of successive secondary combustion Zones leads to greater efficiency.
P.C.T. Pat. No. WO 85/03762 ('762) to Danielsson discloses a combustion apparatus that uses multiple air supply ducts to horizontally inject air into the combustion chamber, leading to turbulence and thereby facilitating more complete combustion. Importantly, '762 discloses an apparatus that uses a plurality of combustion zones. However, through experimentation, it has been found that horizontally fed air is not the most efficient way of establishing a turbulent air environment. Further, in the '762 disclosure, not all zones are used for combustion: only zones one, two and three combust the fuel. The fourth zone does not combust any fuel. The '762 patent uses similar concepts of creating a turbulent air environment to improve combustion. The '762 patent also suffers from the disadvantage of requiring a calibration procedure to adjust the flow of the secondary air supply ducts at the time of installation. This is impractical because each fuel source (i.e. different types of wood or coal) combusts differently, so the apparatus of '762 requires calibration every time a different fuel is used.
U.S. Pat. No. 4,658,801 ('801) to Black discloses a stove comprising three sequential combustion chambers and a top-down window air wash. Patent '801 also discloses the use of stainless steel for the purpose of reflecting heat to maintain heat at its catalytic converter. It is important to note that this is the extent of the stainless steel's use. Patent '801 relies on a catalytic converter between the second and third combustion chambers for complete combustion of the fuel; the disadvantage of this has been discussed above. Further, in practice, the top-down air (counter-acting airflows) results in a build up of combustion products at the bottom of the window, thereby requiring the user to constantly clean the window and thereby defeating the purpose of the air wash.
U.S. Pat. No. 5,341,794 ('794) to Henry et al. discloses the use of SCZs, the use of multiple self-induced air sources, and the use of a top-down air wash. The air supplies create a turbulent air environment and purportedly establish three SCZs; however, because of the close proximity of the air supplies, the disclosed three SCZs actually form one SCZ. Also, as explained hereinabove, the use of a top-down air wash does not work well in practice.
U.S. Pat. No. 4,319,556 ('556) to Schwartz et al. discloses a combustion apparatus with two combustion zones and discloses the use of injected secondary air. Secondary combustion is indeed achieved; however, it is achieved through the use of a catalytic converter. The disadvantage of using a catalytic converter has been discussed hereinabove.
Prior art solid fuel combustion apparatus employ gravity-fed, natural draft chimney systems of at least 4.3 m (14′) of vertical height in order to create the necessary draw or pressure differential to remove exhaust gasses and the products of combustion. This vertical height restriction severely limits the type of building in which solid fuel apparatus may be used due to space and design constraints.
The above disclosures teach similar approaches to generating secondary combustion. The above disclosures either employ secondary combustion by using turbulent air in particular regions of the firebox, or they achieve secondary combustion by using a catalytic converter. There is need, however, for a secondary combustion apparatus that has been designed to optimally burn all types of solid fuel products without relying on a consumable, catalytic converter to reduce emissions and increase efficiency. Further, there is need for a window air wash that keeps the viewing window clean in practice. There is also a need for a self-regulating and self-adjusting secondary combustion apparatus. There is also a need for a clean burning secondary combustion apparatus for solid fuel which incorporates the visually appealing floating flame esthetics whereby the flames appear to float or dance above rather than on the fuel. There is also need for a secondary combustion apparatus that is adapted to burn any solid fuel, including cord wood, wood pellets, or coal. Finally, there is a need for direct vent combustion apparatus that creates its own pressure differential, obviating the need for relatively lengthy chimneys, thereby permitting relatively short and alternatively routed chimney structures for use, for instance, in high density housing.